Designers used to transforming metal parts into plastic might be intrigued by the idea of getting a glossy, weather-resistant, "Class A" surface straight from the injection mold. That's because painting was once the only way to get such good surface quality. A Class A surface straight from the mold would be a tremendous boon to automaking as paint lines typically occupy 50% of a plant's floor space. Moreover, the initial capital investment for such lines is on the order of one-half billion dollars.
Complex geometric parts ranging in size from cell phones to large truck hoods are candidates for a recently developed in-mold decorating (IMD) process. The process coextrudes a UV-absorbent, scratch and chemical-resistant, high-gloss cap layer onto a colored substrate. The resulting material combination out performs paint systems in several areas including surface damage, surface quality, VOC elimination, and recyclability.
IMD is well known for putting graphics, such as logos and model names, directly on the complex 3D parts without secondary operations. A film in either roll or sheet form undergoes sequenced drying, thermoforming, and trimming. Next a back molding step puts the film into the injection-molding tool before the substrate resin injects.
Traditional IMD films, however, couldn't produce Class A finishes nor withstand the rigors of outdoor abuse or the ravages of weather in applications such as automotive body panels. A recent paint-replacement film, Lexan SLX, is tough enough for such applications and has outstanding aesthetics. It has great gloss, UV stability, and resistance to chemicals and scratches. Candidates for the process now include industries such as lawn and garden, recreational vehicles, marine, sporting goods, heavy truck or bus, and agricultural equipment.
Moreover, traditional indoor applications including consumer electronics and cell phones are candidates as well because IMD easily promotes product differentiation. Simple changes in film color or graphics, for example, can provide an easy means of reintroducing an existing part as a new model. In addition, IMD using the Lexan SLX film may also let designers use regrind materials or less-costly commodity-based resins as substrates without losing surface quality or performance.
Current applications for the technique include roof modules on the Smart GmbH (a business unit of DaimlerChrysler AG) Roadster and fenders for Segway Human Transporters. Automakers are also evaluating the film as a potential paint replacement material for exterior body panels.
There are several considerations before designing a part for IMD using Lexan SLX film. For one, part design must account for potential limitations of the manufacturing process. Second, materials must be evaluated for adhesion, physical and aesthetic performance, as well as mold-filling capabilities. Tight process control is a must, especially in the thermoforming and trimming phases. The last but by far most-important consideration is tool design. IMD processes are complex, so proper fit of the formed Lexan SLX skin in the injection mold is paramount. This entails understanding the film shrinkage, thermal expansion of tooling materials, as well as processing conditions.
Understanding the materials used in this process is key to delivering functional parts that are aesthetically pleasing. The film portion of an IMD part delivers the surface aesthetics, surface performance, and in the case of Lexan SLX films, weatherability. Designers must understand how stretching induced during the forming process can affect these properties.
Evaluations of material responses during forming have revealed that the layers of Lexan SLX film generally draw down proportionally (i.e., get thinner) during the forming. Thus there's a minimum cap-layer thickness to maintain weatherability performance. Such boundary conditions should help designers identify the initial film thickness needed for a specific geometry.
In addition, the high glass-transition temperature of Lexan SLX resin prevents the appliqué from significantly stretching during injection molding. The stiff nature of the film also facilitates robotic handling.
Like many engineering thermoplastics, Lexan SLX films need to dry prior to thermoforming to remove moisture absorbed during transportation. Improper drying can degrade thermoforming repeatability and cause part surfaces to pit or blister.
Three main factors dictate substrate selection: adhesion, physical properties, and flow behavior. Substrate color must also be taken into account, as it can affect the look of the final part. Lexan SLX films are based primarily on the polycarbonate (PC) molecular structure; therefore, substrates compatible with PC bond best.
In addition to Lexan, other substrate candidates include polybutylene terephthalate (PBT) and polymer blends such as PC/ABS and PC/PBT. Likewise, for high stiffness, rigidity, and low CTE, Azloy nonwoven chopped glass-fiber/PC mat from Azdel Inc., Shelby, N.C., exhibits good adhesion performance with the film when compression molded. In every case, however, the resin/film combination should be evaluated with caution.
While substrates can provide the right blend of physical and adhesive properties, it's also important for the designer to understand substrate flow behavior. The focus on flow behavior becomes even more heightened in applications where IMD is deployed with existing tools not designed for IMD. Typically a part's wall thickness is optimized for a specific material, flow length, and cycle time. Attempting to introduce a film in the injection tool results in a thinner wall, further hindering flow behavior.
In evaluating a substrate, processing melt temperature plays a key role as well. A low melt temperature, for example, can limit mold-filling capabilities and, most importantly, may reduce adhesion. Too high a melt temperature, on the other hand, degrades physical properties and film color washout. Designers can make a preliminary mold filling analysis for IMD parts by reducing part wall thickness by the thickness of the film.
One of the biggest gaps in today's large-part IMD process is in the infrastructure of film thermoforming. GE Plastic's Polymer Processing Development Center (PPDC) has put a lot of effort into better understanding material requirements as well as processing and design limitations. The center has a large-part IMD cell dedicated to automotive designs and is investigating several other industries as potential IMD candidates.
With increased interest in large IMD parts, several thermoforming-equipment manufacturers have devised more sophisticated equipment. A few OEMs and Tier 1 suppliers have invested in these capabilities to eliminate painting.
There is rapid heat lost during the forming of the thin IMD films. So thermoforming equipment must be optimized for heat management. A common requirement for processing Lexan SLX films is that thermoforming machines be able to transfer films from the heating zone to the forming station in less than 3 sec. In-line and shuttle-type thermoformers can typically handle the task. The limitation for in-line systems is part size and height, while production volumes (below 100,000 units) should be considered before investing in shuttle-type formers.
Another important factor for handling fast-setting films is the ability to pull a vacuum on the parts as the film heats and forms. New heating technology, such as halogen "flash" heaters and material-control systems such as zero gravity, also play a key role in being able to obtain near "Class A" thermoformed surfaces.
Prototyping of plastic parts often takes place on epoxy or wooden tools. But production tools for "Class A" IMD have stringent requirements. Thermoforming production tools for Lexan SLX films should be aluminum. This is necessary because the tool must hit 230°F (110°C), at minimum. Oil or electric cartridges often handle the heating.
"Class A" tool design also restricts the use of vacuum holes on the show surface. They can appear only on nonshow areas or style lines. In addition, the extreme sensitivity of the film-show surface prevents core pulls/actions from being employed in show areas. Otherwise the gap will read through to the first surface and will not be ironed out by the injection-molding process.
Thermoformed parts need a trimming operation to remove excess material. Several trimming technologies are currently available. The distinguishing capabilities are linked to part size, film thickness, part geometry, and production volumes. Typical equipment for 2D trimming includes matched metal, hot and cold knife, and die cutting. 3D geometries typically require a five or six-axis robot wielding laser, ultrasonic knife, or CNC router technologies. Every trimming technique has its pros and cons. But a laser-based system is generally preferred for extremely complicated geometries.
Success of an IMD part often hinges on the fit of the thermoformed film in the injection-molding tool. But other key considerations include a proper gating system, the right wall thickness, the proper tonnage, registration method, and automation plan. Gating accounts for part filling capability, film and color washout, and often part performance. Large parts demand careful use of multiple drops to avoid knit line readthrough to the show surface and keep the part from deforming during thermal cycling. Proper gating design won't cause the color layer to wash out and can often aid in the registration of the film.
Walls with a uniform thickness in the tool will help produce a uniform aesthetic surface. A sudden change in substrate thickness translates into a defect on the part's first surface. The film registration must be robust to repeatedly deliver good IMD parts. Typical registration methods include part geometry, mechanical aids, electrostatic charges, and vacuum.
Part and tooling design
IMD technology typically handles parts requiring specific functional or aesthetical performance. A key first step is the determination of film coverage. Success or failure often rests on how much of the part the film will cover. The appliqué molding process allows a great deal of freedom in picking colors, graphics, and textures, but there are still limits to what is practical.
At the top of the list of challenges is edge wrapping. Edge wrapping largely depends on processing technique and part size. Key parameters that should be established before designing tools include: part geometry, edge geometry, number of sides to wrap, degree of wrap, and distance of wrap.
The edge wrapping of a part typically implies the presence of undercuts in both thermoforming and injection-molding tools. Undercuts are often relieved via a mechanical motion of the tool or part thereof. The motion of these movable cores should aid in the release of the part and then reassemble to reform the part surface, leading to the formation of lifter lines.
With high-gloss aesthetic films it's important to evaluate the effect of lifter lines on a show surface. Studies have revealed that lifter lines produced by moving cores in the tooling will read through to the surface of the appliqué. These surface defects can't be removed during the IMD process. To minimize lifter lines tools can be designed so the core action aligns with a highly defined style line or nonshow area.
Designing a tool without core action is also an option, but it complicates the molding of undercuts and edge wrapping a part. One technique employed with coreless tools is pivoting the entire tool to help release the part. It is also advisable to keep vacuum holes off part show surfaces unless disguised by a style line or extreme curvature. Texturing of the thermoforming tool will also affect the look of the final part.
The final step in the process is the back molding of the film. This also affects the quality of the part surface. For example, readthrough, which would be an issue with standard injection molding, can be highlighted with the addition of a glossy film. Key elements such as ribs, bosses, and any sudden change in the thickness of the part will also read through as a mark caused by nonhomogeneous cooling and material shrinkage. In addition, thin flat parts with an appliqué covering the entire front surface may warp. Offsetting A and B-half mold temperatures helps minimize warpage.
It is important for designers to take into account that the higher coefficient of thermal expansion of plastic parts made via IMD will be less dimensionally stable than those made of metal. However, it is possible to design the part so critical dimensions can be fixed, while allowing the rest of the part to expand or contract as temperatures change. Another technique employed is to let the part bow and flex. All in all, however, final part surface quality is a key feature that lets Lexan SLX films compete with painted surfaces; parts typically possess a glossier (lower dullness), smoother (low orange peel), mirror-like surface.
Azdel Inc., Shelby, N.C., (800) 635-7021, www.azdel.com
GE Advanced Materials, Plastics, Pittsfield, Mass., (413) 448-5800, www.geadvancedmaterials.com
Top 10 design considerations for IMD
Do the production volumes justify it?
Can the required aesthetics be reproduced with films?
How much of the surface can be covered via the IMD process?
What is the minimum film thickness for this application?
What substrate is suitable for the application?
What information does the designer need to properly design the forming, trimming, and injection tools?
What trimming methodology applies?
What injection-mold-tooling factors (wall thickness, gating, registration) must change to implement IMD?
What infrastructure is in place to help manufacture such a design?
How does the performance of the part change with the addition of film?